1. Field of Invention
This invention relates to a cast wear resistant martensitic type iron base alloy containing high carbon, high molybdenum and/or tungsten and other alloy elements like vanadium and niobium, to improve hot hardness and wear resistance for making internal combustion engine valve seat inserts (VSI), where carbon is in the range of 2.05-3.60 wt. % and molybdenum plus tungsten is in the range of 11.0-25.0 wt %. The inventive alloy is especially useful to make exhaust VSls used in heavy duty internal combustion engines where the working conditions are severe enough to demand for VSI alloys with excellent wear resistance. On the other hand, this alloy also relates to high carbon and high alloy type steels and cast irons. In a further aspect, this invention relates to components made from such alloys, either cast or hardfaced. Alternatively, components made of such alloys may be made by conventional powder metallurgy methods either by cold pressing and sintering or by hot pressing at elevated pressures for wear resistant applications.
2. Prior Art
High temperature wear resistance is the most important property for exhaust VSI alloys used in internal combustion engines, where the average exhaust VSI seat surface working temperature is 550-950° F. Hot hardness or high temperature hardness is one of the key factors affecting wear resistance of exhaust VSI materials. Iron, nickel and cobalt base alloys are the most common alloy families used for making exhaust VSIs in diesel or dry fuel internal combustion engines. High carbon and high chromium type nickel base alloys were developed in 1970s as disclosed in U.S. Pat. No. 4,075,999 to replace Stellite type cobalt base alloy as exhaust VSI materials, and these nickel base alloys are still in use in less demanding engine applications. Because of their relatively lower cost, iron base alloys, like M2 tool steel, are also found their applications as VSI materials in many diesel engines where the working conditions are within the performance range of these iron base alloys. In the early 1990s, a high speed type iron base alloy was developed and the subject of U.S. Pat. No. 5,674,449, to fill the gap between nickel base alloys and cobalt base alloys as exhaust VSI material. It was the first VSI alloy utilizing alloy element niobium with higher amount of molybdenum, tungsten and chromium, etc. to improve wear and oxidation resistance. Because of its high performance-to-cost ratio, this alloy has been widely used as an exhaust VSI material in diesel engine industry. Wear resistant alloy carbides with tempered martensitic matrix and adequate oxidation resistance are the essential factors for good wear resistance of the iron base alloy.
U.S. Pat. No. 5,674,449 discloses an alloy in which carbon is 1.6-2.0%, chromium 6.0-9.0%, the total of molybdenum plus tungsten is 11.0-14.0%, vanadium 1.0-8.0%, niobium 0.5-5.0%, cobalt 2.0-12.0%, and iron being balance.
U.S. Pat. No. 6,702,905 discloses an iron base alloy as diesel engine VSI material. This alloy contains carbon 1.2-1.8%, boron 0.005-0.5%, vanadium 0.7-1.5%, chromium 7-11%, niobium 1-3.5%, molybdenum 6-11%, and the balance including iron and incidental impurities.
U.S. Pat. No. 6,436,338 discloses another iron base alloy for diesel engine VSI applications. The alloy composes of carbon 1.1-1.4%, chromium 11-14.5%, molybdenum 4.75-6.25%, tungsten 3.5-4.5%, cobalt 0-3%, niobium 1.5-2.5%, anadium 1-1.75%, copper 0-2.5%, silicon 0-1%, nickel 0-0.8%, iron being the balance with impurities.
U.S. Pat. No. 6,866,816 discloses an austenitic type iron base alloy with good corrosion resistance. The chemical composition of the alloy is 0.7-2.4% carbon, 1.5-4.0% silicon, 3.0-9.0% chromium, less than 6.0% manganese, 5.0-20.0% molybdenum and tungsten, the total of vanadium and niobium 0-4.0%, titanium 0-1.5%, aluminum 0.01-0.5%, nickel 12.0-25.0%, copper 0-3.0%, and at least 45.0% iron.
U.S. application Ser. No. 10/074,068 discloses another type of wear resistant alloy containing residual austenite as VSI material. This alloy contains 2.0-4.0% carbon, 3.0-9.0% chromium, 0.0-4.0% manganese, 5.0-15.0% molybdenum, 0.0-6.0% tungsten, 0.0-6.0% vanadium, 0.0-4.0 niobium, 7.0-15.0% nickel, 0.0-6.0% cobalt, and the balance being iron with impurities.
However, the VSI material disclosed in these patents do not exhibit a sufficiently high wear resistance required for many new internal combustion engines which have higher power output and combustion pressure and produce less emission. Also, the hot hardness and wear resistance of the VSI material disclosed in these patents is not high enough for these new engines. Although cobalt base alloys like Stellite® 3 or Tribaloy© T-400 offer adequate hot hardness and wear resistance as VSI materials in certain demanding applications, the high cost of cobalt element limits these cobalt base alloys to be widely accepted in the engine industry. On the other hand, as the alloy content in iron base alloys reaches a fairly high level, like in U.S. Pat. No. 5,674,449, manufacturing cost becomes a significant factor to determine if the iron base alloys will be competitive in performance to cost ratio compared to cobalt base alloys. The casting VSI manufacturing cost is affected by casting scrap rate, alloy heat treatment cost, machining ability, and inspection of these casting wear resistant alloys etc. For a given casting technique, the casting scrap rate and heat treatment response are directly affected by alloy chemical compositions. Though VSI machining ability is also affected by the alloy chemical composition it is difficult to achieve good wear resistance and good machining ability at the same time in this group of alloy.